Method of making cermet resistance element and terminal connections therefor



METHOD OF MAK June 4, 1968 L, JR, ET AL 3,386,165

ING CERMET RESISTANCE ELEMENT AND TERMINAL CONNECTIONS THEREF Original Filed Feb. 1, 63

FIG. 3 FIG. 4

INVENTORS DONALD A. BRUHLJr. WILLIAM E. COUNTS United States Patent 0 4 Claims. (QB. 29-621) ABSTRACT 0F THE DISCLQSURE Formation of a low resistance junction between a conductor lead and a cermet resistance element deposited on a ceramic substrate including the formation of a fused metal-glass layer between the interface portions of the conductor lead and the adjacent cermet resistance element deposited on the substrate, the metal-glass layer having substantially greater proportion by weight of met-a1 constituent than glass constituent.

The present invention relates to electrical resistance devices of the type utilizing a resistance element in the form of a thin layer of cermet resistance material disposed on a non-conductive base member and is more particularly related to terminal and tap connections to such a resistance element.

This is a division of our copending application Ser. No. 255,575, filed Feb. l, 1963, entitled Cermet Resistance Element and Terminal Connections Therefor, now abandoned.

Cermet type resistance materials are formed from composite mixtures of finely divided glass particles and finely divided metal particles which are applied to a non-conductive base member and fired to a temperature below the melting point of the metal but sufiicient to melt the glass particles and to fuse the mixture into a conglomerate mass. It has been found best to use noble metals in the mixture which tends to resist oxidation at the temperatures required to melt the glass and to fuse the mixture. The fused mixture produces a smooth, continuous surface that is resistant to high humidity and fungus. Because of the hard, smooth surface and durability of the resistance element, it is especially applicable to variable resistance devices such as potentiometers or rheostats which utilize a movable contact that must be traversed over the resistant surface.

In the finished resistance element, the noble metal particles are uniformly dispersed throughout the solidified glass and form a semi-conductive path through the material. While the exact electrical phenomena existing within the resistance material is not fully understood, it is believed that the metal particles are slightly spaced from each other or are only slightly touching throughout the mixture to produce a high resistance. A number of typical cermet resistance films and the method of making these films are described in US. Patent Nos. 2,950,995 and 2,950,996 issued in the name of Thomas M. Place, Sr., et a1. and assigned to the same assignee as the present invention.

One of the difficulties associated with .cermet type resistance elements and the manufacture thereof is the attainment of suitable terminal and tap connections with the resistance element without greatly affecting the resistance characteristics of the cermet element in the region of terminal junction. It is desirable to form the terminal or tap connections at the same time that the resistance mixture is fired so that the metal taps or terminals are 3,386,165 Patented June 4, 1968 actually fused to the resistance element in the finished product. This not only lends physical support to the taps or terminals but also eliminates the need for later attachment of the terminals by welding, soldering or other process.

However, when the cermet resistance material is fired directly onto the terminal or tap conductor member, it has been difficult to maintain reasonable terminal resistance values, and almost impossible to obtain the same terminal resistance value for the elements on a uniformly acceptable basis from batch to batch. It has been noted in certain defective resistance elements that the noble metal particles of the cermet mixture apparently migrates from the region of the terminal or tap connection during the firing or fusion operation and thereby leaves a void or partial void in this region.

This has the effect of greatly increasing the terminal resistance or making the element a non-conductor in this region. While it is not completely understood why the metal particles seem to migrate from this region during the firing operation, or why different results are obtained on a batch-to-batch basis, it is very evident that the tendency for the metal particles within the material to migrate contributes substantially to the increase in resistance and is one of the major reasons why these terminal connections cannot be manufactured on a reasonably uniform basis.

When the terminal leads, in a resistor of this type, are connected directly to the cermet resistance element there is a tendency for the conductor lead and cermet resistance material to separate or part, when the application of the resistor is a type wherein the temperature varies over a great range, due to the difference in coefficient of expansion between these materials. This, of course, creates increasingly higher resistance at the terminal junction and makes the resistance device subject to failure.

It is an object of the present invention to provide a method for forming an improved terminal or tap connection between a conductor member and a cermet resistance element that may be employed on a production basis to provide terminal connections having a relatively low resistance value which is uniformly the same from resistor to resistor.

Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In carrying out the object of the present invention, there is provided an improved method for making a low resistance junction between a conductor lead and a cermet resistance element including the formation of a fused metal-glass layer between the interface portions of the conductor lead and the adjacent cermet resistance element, the layer having a substantially greater proportion by weight of metal constituent than glass constituent.

For a better understanding of these and further aspects of the invention, reference may be had to the accompanying drawing in which:

FIG. 1 is a perspective view of a resistance element embodying the present invention;

FIG. 2 is an enlarged partial top view of a junction between a conductor lead and a cermet resistance film, illustrating somewhat schematically, the non-conductive region that is created by the noble metal particle migration within the cermet when the cermet material is applied directly to a metal conductor lead;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2 further illustrating the region of non-conductivity created by the migration of metal particles in a cermet resistance film when applied directly to a metal conductor lead; and

FIG. 4 is a greatly enlarged cross-sectional view taken along line 4-4 of FIG. 1 illustrating the metal-glass mixture interposed between the interface surfaces of the conductor lead and the cermet resistance mixture.

Referring now to FIG. 1 of the drawing, there is shown a resistor element of the type adapted for use in the variable resistance art. The resistor element comprises a substrate base member 2 of dielectric material which may be in the form of a circular disc or water. The member is usually formed of an unglazed homogeneous, non-porous ceramic material, such as steatite or alumina or other nonconductor material well known in the art. While the illustrated resistance device is in the form of a circular disc of the type used in rotary turn potentiometers or rotary variable resistors, it will be understood that the present invention is not limited to resistance devices of this particular shape and that the invention is equally applicable to the manufacture of resistance devices of rectangular shape or any other suitable configuration.

In conventional practice, the substrate or base member 2 is molded, fired, and ground or lapped to provide a smooth, relatively fiat surface 2a for supporting a resistance film or element 3 thereon. The resistance film 3 forms the conductive path for an electrical current which is applied to the resistance film by means of conductor leads. In the illustrated embodiments of the invention, there are provided a plurality of conductor ribbons or leads 4, 5, and 6 through which an electrical current may be applied to the resistance film 3. Conductor leads 4 and 5 are terminal connections with the element, and conductor ribbon 6 illustrates a center tap connection to the resistance element. As hereinafter used in the specification and claims, the terms conductor terminals or conductor leads are meant to include both end terminal connections or center tap connections commonly employed in the resistor or variable resistor field.

Conductor terminals 4, 5 and 6 are formed of a metal or alloy having a relatively high resistance to oxidation even at the elevated temperatures required to fuse the cermet resistance film 3. When the terminal members are imbedded in the base member 2, as in the illustrated embodiment of the invention, it is also important that the metal terminal member have a coefficient of expansion closely matching that of the ceramic material from which the base member 2 is formed. The noble metals, because of their resistance to oxidation, have been found best suited for this purpose and various alloys of these noble metals of gold, silver, platinum, palladium or rhodium have been formed to provide a coefiicient of expansion that matches, to some extent, that of the ceramic base member 2. One alloy found very suitable for this purpose is a palladium alloy manufactured and sold by the J. M. Ney Company under the tradename Paliney.

The resistance element or layer 3 comprises a fused layer formed from a mixture of non-conductive glass binder material with minute particles of noble metal or metal alloy dispersed throughout the element and having trace amounts of certain other materials added When desired. Particular examples of cermet materials and the method of manufacture thereof are fully explained in the aforementioned Place et al. Patent Nos. 2,950,995 and 2,950,996. The mixture of glass and metal particles, after fusion at a temperature below that of the metal constituent, forms a continuous element having a hard, smooth, glassy surface. The glass-metal cermet mixture is predominantly glass with relatively small percentages of metal, depending upon the particular value of resistance desired.

The cermet material is applied to the base member in a configuration desired for the particular application of the resistance device. In the disclosed disc-type element, the film or layer is circular or arc shaped and may be ap plied to the base member 2 by any suitable operation well known in the art, such as brushing, spraying, stencilling or silk screening.

After the cermet film or layer 3 has been deposited upon the surface 2a of the base member 2, the base and layer are, preferably, permitted to dry in circulating warm air for a short period. The unit is then fired in a furnace or conventional ceramic kiln, in a clean atmosphere. The furnace or kiln raises the temperature of the unit to the fusion temperature of the glass constituents of the cermet mixture. The purpose of the firing operation is to solidify the glass into a continuous glassy phase, with the metal particles uniformly dispersed throughout the material, without melting the metal particles and without producing bubbles or blisters on the surface of the layer 3. The temperature to which the unit is fired is critical in that firing at too low a temperature results in failure to achieve a continuous glassy phase in which the resistance material has a hard, smooth surface. Firing too high produces bubbies or blisters in the cermet materials and causes the metal particles therein to agglomerate and thereby completely interrupting the conduction path in the film. The required firing temperature depends upon the particular type of glass used in forming the mixture. Various firing temperatures for various types of cermet materials are well described in the aforementioned Place Patents 2,950,- 995 and 2,950,996 and a further description thereof is not believed necessary for a full understanding of the present invention.

As previously mentioned, the attachment of the noble metal conductor leads or terminals 4, 5 and 6 to the cermet resistance layer causes, under some conditions, a migration of the minute metal particles in the cermet layer during the above-described fusion operation. The migration problem and its effect may be more clearly understood upon reference to FIGS. 2 and 3, which show a partial section of a cermet layer fused directly over a conductor lead 6a imbedded in a base member 2. It will be understood, however, that the migration problem occurs not only when the noble metal conductor leads are imbedded in the steatite base member 2, as in the preferred embodiment illustrated in the drawing, but also when the leads are attached to the base and merely in contact with the cermet material during the fusing operation.

As will be noted in FIGS. 2 and 3, all of the metal particles in the region 7 of the cermet layer above the imbedded conductor lead (designated 6a in the drawing) have migrated from this region '7 and leave a void or partial void that breaks, or at least greatly increases the resistance, in the conductive path from the lead to the cermet layer. While the cause of this migration is not completely understood, it is believed to be the result of the heat conductivity of the noble metal lead or due to a creation of an electrostatic charge on the noble metal lead. It is also not completely certain that the noble metal particles migrate away fro-m the conductor lead or migrate toward the conductor lead or whether the direction of migration (i.e., away from or toward the conductor lead) is always the same for different metal leads or for different noble metal particles in the cermet mixture. It has been observed, in a cermet mixture of approximately 95 percent glass and having a metal constituent comprising approximately five percent gold-palladium alloy when applied over a Paliney conductor lead, that the metal particles were apparently attracted toward the conductor lead and collected thereon. Regardless of the direction of migration, the effect is to produce a void or partially void region 7 in the cermet film 3 adjacent the conductor lead 6a which breaks the conductive path or greatly increases the resistance in this region.

In addition to the creation of a non-conductive void, the attachment of the conductor lead directly to the cermet material sometimes is subject to separation, when the device is used under conditions where the temperature varies over a great range. That is, because the noble metal conductor lead usually has a different coefiicient of expansion than that of the cermet resistance material, which is largely glass, the bond between the two is subject to separation when the temperature changes to any great extent. The separation at the junction of these two elements of the device creates a junction resistance that may be very undesirable in the particular electrical apparatus or electrical circuitry in which the resistor is used.

As will be hereinafter described, the present invention is directed to an improved junction or terminal connection that overcomes the above-described disadvantages. Referring now to FIG. 4, which shows a cross section of the preferred embodiment of the invention, it will be noted that a thin layer 8 of fused noble metal-glass mixture is interposed between the interface surfaces of the conductor lead 6 and the cermet resistance layer 3. The layer or covering 8 comprises a noble metal-glass mixture having a relatively high concentration of metal constituent compared to the glass constituent so that the layer, when fused over the noble metal conductor, provides an electrically conductive path from the conductor to the cermet resistance layer.

The particular composition of the glass utilized is not critical to the practice of the invention and various changes in the composition thereof can be made to alter the fusion temperature, coefiicient of thermal expansion, fluidity, solubility, etc., in order to provide desirable characteristics. The glass used for this thin layer 8 is preferably similar to the glass utilized in the composition of the cermet resistance element except that it is best that it have a slightly higher melting or fusion temperature than that used for the cermet material in order to prevent the layer from flowing too freely when the cermet resistance film is fired. The composition of a glass which has been used in practice of the invention is given below, as illustrative, but is in no way intended to be restrictive to the composition used in the glass-metal mixture:

Glass A Raw Batch Composition: Parts Red lead 67 Zinc oxide 5 Boric acid l6 Flint l8 Ultrox 4 Fused Composition: Approx. percent Lead oxide 66 Zinc oxide 5 Boric oxide 9 Silica l8 Zirconium oxide t 2 Preferably the glass is formed by mixing the raw batch constituents and melting the mixture to form a clear fluid glass. The fluid glass is then quenched in a cold water bath and then dried and ground to a very fine powder of approximately 325 mesh in size suitable for mixing with the metal constituent of the glass-metal mixture.

The noble metal constituent of this layer 8 is non-reactive with the glass constituent at the fusion temperature of the glass and may comprise any of the noble metals including gold, silver, palladium, platinum, rhodium and/ or various alloys of these metals. The noble-metal and glass are usually mixed in finely powdered form and it is preferable that the metal constituent of the mixture contain particles of metal of less than 35-40 microns in size. A liquid carrier is added to the mixture to permit application of the mixture in a thin layer onto the conductor lead surfaces by any suitable means such as brushing, spraying, stencilling or silk screening. The type of liquid carrier used is not critical-to the invention, and any suitable carrier such as toluol, xylol, oil, lacquer mixtures or even water, may be used. The quantity of liquid carrier used is selected to give the proper viscosity for the particular method employed to apply the mixture onto the surfaces of the conductor lead.

After the layer 8 is applied to the proper surface of the metal conductor lead (such as on the upper surface of the conductor lead 6 of FIG. 4) it is fired in a suitable furnace to a temperature sufiicient to melt the glass but below the melting point of the metal constituent of the mixture. The firing temperature depends upon the particuiar type of glass used. It has been found that a temperature of approximately 1400 F. to 1500 F. for a short period results in complete fusion of the noble metalglass mixture using a glass having a composition approximately the same as that of the aforementioned glass A.

In actual practice, a noble metal-glass mixture containing percent by weight of gold particles in finely divided form mixed with a glass of the type previously set forth as glass A, has been fused in a very thin layer over a palladium alloy conductor lead. The layer or film of metal-glass mixture, when fused, was Only several thousandths of an inch thick and, yet, when the cermet mixture was applied over this film, there was no tendency for the metal particles in the cermet material to migrate during the fusing operation of the cermet material. In addition to the above-mentioned gold-glass mixture, various other noble metal-glass mixtures have been tried using silver, palladium, platinum and rhodium, as Well as alloys of these metals, all with satisfactory results.

In other tests the percentage by Weight of the noble metal constituent was reduced to slightly more than 50 percent of the total Weight of the mixture, and a junction was provided having extremely low or negligible terminal resistance. This is particularly true of the gold, platinum and palladium metals and alloys of these metals when mixed with the glass constituent and when ground to extremely small particle size. However, when the metal constituent is much less by weight than that of the glass constituent, it has been found difficult to provide uniform results and the resistance of the junction begins to increase. It is believed that the metal-glass mixture then begins to take on the characteristics of a resistor. Therefore, the particular percentage of the noble metal and glass constituents used is important, in order to assure that the resulting layer will be a conductor, and it is preferable to have the metal comprise the major constituent of the mixture by weight with a relatively smaller amount of glass. It has been found that the best range of proportions for the finished roduct will include approximately five percent to slightly less than 50 percent glass by weight and slightly more than 50 percent to 95 percent noble metal by weight. These proportions of metal and glass, when fused, result in a homogeneous layer of material having electrical conductivity very close to that of the pure metal constituent.

The above-described mixture is applied to the entire surface of the conductor lead that is exposed, or that could otherwise come onto contact with the resistant layer 3. While, in FIG. 4 the layer 8 only covers the upper surface of the conductor lead 6, it will be understood that, if the lead were not imbedded into the base member 2 but were to be joined with the cermet layer in another manner, such, as for example, being imbedded completely within the cermet resistance layer, then the layer 8 of glass-metal conductive material would cover all those surfaces of the conductor lead between the cermet material and the lead. After the layer of noble metalglass mixture is applied to the surface of the conductor lead, it is fired to a temperature sufficient to fuse the glass constituent. The resulting layer is a hard, spongy mass that is a good electrical conductor and securely bonded to the surface of the conductor lead.

The cermet resistance material is then applied over the interface layer 8 and fired, as described in the aforementioned Place et al. patents, with the cermet material in contact with the layer 8 rather than being in direct contact with the conductor lead 6. It has been found that,

when this fused layer of predominantly noble metal and glass is first fused between the metal conductor lead and the cermet resistance layer, there is no migration of the particles within the cermet material during the fusing op eration of the cermet resistance material and therefore no non-conductive or partially non-conductive void in the junction area. Using this method wherein the homogeneous layer of predominantly metal and glass mixture is fused between the conductor lead and the cermet film prior to the fusion operation of the cermet film, it is possible to make terminal connections having a reasonably low terminal resistance and on a reasonably consistent basis from batch to batch.

The spongy nature of this layer between the cermet material and the conductor lead serves to absorb the variations in expansion between the conductor lead and the cermet resistance film thereby greatly reducing the tendency for these elements to separate under widely varying temperature conditions. Thus it is possible to employ the resulting product in devices to be used over widely varying ranges of temperature.

While in accordance with the patent statutes there is described What at present is considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A method of making a resistance element comprising the steps of:

providing a non-conductive substrate member having a substantially smooth surface thereon;

providing at least one noble metal conductor member;

providing a homogeneous mixture of finely divided noble metal particles and finely divided glass particles, said mixture comprising a greater portion by weight of noble metal constituent than glass constituent;

applying a thin layer of said noble metabglass mixture to a surface of said noble metal conductor member;

firing said metal conductor member having said metalglass mixture thereon above the melting point of said glass constituent of said mixture but below the melting point of said metal constituent thereof thereby to fuse a thin homogeneous layer of metal-glass mixture to said surface of said conductor member;

applying :a layer of cermet resistance material on said surface of said substrate base member;

supporting said conductor member relative to said substrate base member with said layer of metal-glass mixture on said conductor member in contact with said cermet resistance material; and

firing said cermet material on said base member to a temperature lower than the melting point of said metal constituent of said cermet mixture and exceeding the melting point of the glass constituent of said cermet mixture thereby to provide a thin layer of resistance material attached to the surface of said base member and to said layer of metal-glass mixture on said surface of said conductor member.

2. A method of making a resistance element comprising the steps of:

providing a non-conductive substrate member having a substantially smooth surface thereon;

providing :at least one noble metal conductor member;

providing a homogeneous mixture of finely divided noble-metal particles and finely divided glass particles, said mixture comprising a greater proportion by weight of noble metal constituent than glass constituent;

applying a thin layer of said noble metal-glass mixture to a surface of said noble metal conductor member; firing said metal conductor member having said metalglass mixture thereon above the melting point of said glass constituent of said mixture but below the melting point of said metal constituent thereof thereby to fuse a thin homogeneous layer of metal-glass mixture to said surface of said conductor member;

applying a layer of cermet resistance material on said surface of said substrate base member; said cermet resistance material having a glass constituent with a lower melting point than said glass constituent of said metal-glass mixture;

supporting said conductor member relative to substrate base member with said layer of metal-glass mixture on said conductor member in contact with said cermet resistance material; and

firing said cermet material on said base member to a temperature lower than the melting point of said metal constituent of said cermet mixture and lower than the melting point of said glass constituent of said metal-glass mixture but exceeding the melting point of said glass constituent of said cermet mixture thereby to provide a thin layer of resistance material attached to the surface of said base member and to said layer of metal-glass mixture on said surface of said conductor member.

3. A method of making a resistance element comprising the steps of:

providing a non-conductive substrate member having a longitudinal noble metal conductor member imbedded therein and having a surface thereof exposed on said smooth surface of said substrate member;

providin a homogeneous mixture of finely divided noble metal particles and finely divided glass particles, said mixture comprising a greater proportion by weight of noble metal constituent than glass constituent;

applying a thin layer of said noble metal-glass mixture to said exposed surface of said noble metal conductor member;

firing said substrate member having said metal-glass mixture on said surface of said noble metal conductor member above the melting point of said glass constituent of said mixture but below the melting point of said metal constituent thereof in order to fuse a thin homogeneous layer of metal-glass mixture over said exposed surface of conductor member;

applying a layer of cermet resistance material on said surface of said substrate base member overlying said thin layer of said metal-glass mixture fused over said conductor member; and

firing said substrate member having said cermet resistance material thereon to a temperature lower than the melting point of said metal constituent of said cermet mixture and exceeding the melting point of said glass constituent of said cermet mixture thereby to provide a thin layer of resistance material attached to the surface of said base member and to said layer of metal-glass mixture over said conductor member.

4. A method of making a resistance element comprising the steps of:

providing a non-conductive substrate member having a substantially smooth surface thereon;

providing at least one metal conductor member;

providing a homogeneous mixture of finely divided noble metal particles and finely divided glass particles, said mixture comprising about -95 percent by weight of noble metal constituent and about 5-50 percent by weight glass constituent;

applying a thin layer of said noble-metal glass mixture to a surface of said noble metal conductor member;

firing said metal conductor member having said noble metal-glass mixture thereon above the melting point of said glass constituent of said mixture but below the melting point of said metal constituent thereof thereby to fuse a thin homogeneous layer of metalglass mixture to said surface of said conductor member;

3,886,165 9 10 applying a layer of cermet resistance material on said base member and to said layer of metal-glass mixture surface of said substrate base member; on said surface of said conductor member. supporting said conductor member relative to said substrate base member with said layer of metal-glass References cued mixture on said conductor member in contact with 5 UNITED TA PATENTS said cermet resistance material; and 2,489,409 11/1949 Green et al. 29-15559 X firing said cermet material on said base member to 21 2,364,926 12/1958 P itiki 333 322 X temperature lower than the melting point of said 2,950,995 8/1960 Place et al 338308X metal constituent of said cermet mixture and ex- 3,056,937 10/1962 Pritikin 29-155] ceeding the melting point of the glass constituent of 10 said cermet mixture thereby to provide a thin layer JOHN CAMPBELL Prlmary Exammer' of resistance material attached to the surface of said CLINE, Assistant Examiner- 

